Liquid propellant rocket engines, commonly utilize bi-propellant coaxial injection elements or the like for injection and combustion of oxidizer and fuel in a combustion chamber. Hundreds of injector elements may exist in supplying propellant to a single combustion chamber.
Referring now to FIG. 1, a close-up cross-sectional view of a traditional coaxial injector element with a flame-holding zone is shown. Coaxial elements typically inject a first propellant through a central post 10 and a second propellant through a surrounding annulus 12. The propellants being oxidizer and fuel. Shear flow created at a boundary 14 between the fuel and the oxidizer is utilized to atomize and mix the propellants prior to combustion in a reaction zone 16 of a combustion chamber 17.
A mixture of oxidizer and fuel exists in the recirculating wake in an area at the end of a post tip 18 of the central post 10, which is forms a flame-holding zone 20. The flame-holding zone, at the end of the post 10, has low axial velocities, recirculating flow, and a very ignitable mixture ratio. Once propellants within this zone are ignited, which can be difficult, the combustion system can be operated at mixture ratios much lower than the well-mixed flammability limit of the propellants.
When oxidizer is flowing through the central post 10 and fuel is flowing through the surrounding annulus 12 a central zone 22 of mixture ratios (oxidizer to fuel) exist ranging from infinity near flow of the oxidizer and approaching zero near the flow of fuel or boundary 14. The central zone 22 is surrounded by an annular zone 24 having a low mixture ratio, approaching zero. The opposite exists when fuel is flowing through the central post 10 and oxidizer is flowing through the surrounding annulus 12.
An ignition system (not shown) is typically coupled to the combustion chamber and may include one or more ignition sources (i.e. spark igniters) that are located downstream of an injector face 25, which are used in igniting the mixture of propellants within the combustion chamber 17. Ignition of the flame-holding zone 20 is important for combustion zone stability and essential for combustors that operate at overall mixture ratios either lower or higher than the well mixed flammability limits for the propellant combination. In order to ignite the flame-holding zone 20 ignition from an ignition source needs to propagate from the source across the annulus fuel flow 12 to the flame-holding zone 20. When multiple elements exist the ignition may have to propagate through multiple zones having high and low mixture ratios to ignite each corresponding flame-holding zone.
In addition to the existing high and low mixture ratios (outside of flammability limits), which are difficult to propagate combustion therethrough, propellant injection velocities are typically high near the injector face, sometimes exceeding flame propagation speeds. The combination of high and low mixture ratios and high propellant velocities, results in difficult to control and unreliable ignition propagation.
In order to ignite the flame-holding zone, from a location downstream of the injector face, propellant injection flow rates must be slowed down or reversed. A pressure surge or “pop” in the combustion chamber accomplishes this and allows the combustion process to propagate up to the flame-holding zone 20 at the tip of the central post 10. A pop occurs from ignition of an undesirable accumulation of unburned propellants within the combustion chamber 17. Once these accumulated propellants in the combustion chamber 17 are ignited, a pressure surge and a temperature spike are created. The pressure surge slows down and sometimes even reverses the injector flow and allows propagation through the slower moving propellants to the flame-holding zone 20. Under normal operating conditions, once the flame-holding zone 20 is ignited it remains ignited.
These temperature spikes and pops over time cause degradation of turbine components due to the higher operating temperatures and thermal stresses. Turbine life is directly related to operational gas temperatures.
Referring now to FIG. 2, a flammability plot of mixed oxidizer/fuel temperature versus mixture ratio is shown for a traditional bi-propellant coaxial injector. Curve 26 represents a border between a nonflammable region 27 and a flammable region 28. Typical turbine drive combustors for liquid rocket engines operate at very low mixture ratios. These low mixture ratios can sometimes be lower than well-mixed flammability limits for a particular propellant combination. This low mixture provides low temperature gases to drive the turbine. Generally, when mixture ratios are increased sufficiently to propagate combustion from the ignition source to ignite the flame-holding zone, gas temperatures are too high for turbine survivability during steady state operation. Typically, operating in temperatures above normal operating conditions or steady state operation, depending upon the turbine and the operating conditions, is undesirable due to material strengths at elevated temperatures and thermal gradients that cause expansion and strain on engine components, thereby reducing engine operating life.
For example, a preburner design used on a space shuttle main engine may operate within a temperature range of 1000° F. to 1500° F. with a mixture ratio of 0.6 to 0.9 of oxygen to fuel. The flammability mixture ratio limit for O2/H2 is approximately 1.2 of oxygen to fuel at −200° F., which has a corresponding and resulting combustion gas temperature of 2200° F. that is too high for survivability of the turbine.
It has been suggested to increase reliability of flame-holding zone ignition is to use a higher mixture ratio during ignition and then reduce the mixture ratio following successful ignition for mainstage/steady state operation. Unfortunately, this results in undesirable temperature during start, which still leads to reduced turbine life.
It is therefore desirable for increased turbine operating life to provide a bi-propellant injector that provides ignition of the flame-holding zone without the typical accompanying requirements and associated disadvantages of high mixture ratio and resulting high turbine temperatures.